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Published November 4, 2020 | Supplemental Material
Journal Article Open

Mixed-Valent Diiron µ-Carbyne, µ-Hydride Complexes: Implications for Nitrogenase


Binding of N₂ by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e⁻ and 4 H⁺ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (P₆ArC)Fe₂(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 e⁻ transfer which bear some resemblance to a pair of E_n and E_(n+2) states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe₂(μ-H)}₂ state of FeMoco (E₄(4H)), the data and analysis presented herein suggest that the hydride ligands in E₄(4H) bridge isovalent (most probably Fe^(III)) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P₆ArC)Fe₂(μ-H)]⁺¹ and [(P₆ArC)Fe₂(μ-H)]⁻¹ by pulse EPR revealed that redox chemistry induces significant changes in Fe–C covalency (−50% upon 2 e⁻ reduction), a conclusion further supported by X-ray absorption spectroscopy, ⁵⁷Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.

Additional Information

© 2020 American Chemical Society. Received: May 31, 2020; Published: September 25, 2020. We are grateful to the NSF for funding (CHE-1905320 to T.A. and an NSF Graduate Research Fellowship to C.H.A.). We thank Prof. Jonas C. Peters and Prof. Ryan G. Hadt for insightful discussions and Prof. Peters for the use of his group's Mössbauer spectrometer. We thank Michael Takase and Lawrence Henling for assistance with X-ray crystallography and David VanderVelde for assistance with NMR spectroscopy. Magnetic data was acquired at the University of California, Los Angeles with assistance from Dr. Ignacio Martini on a Quantum Design MPMS3 SQUID Magnetometer supported by the NSF (MRI-1625776). Support has been provided for the Caltech EPR Facility via an NSF grant (1531940) and the Dow Next Generation Educator Fund, which is also acknowledged for support of the X-ray diffraction and NMR instrumentation. The computations presented here were conducted on the Caltech High Performance Cluster partially supported by a grant from the Gordon and Betty Moore Foundation. The authors declare no competing financial interest.

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